Small vehicles such as snowmobiles may typically have simple electrical systems consisting of a engine driven alternator, usually the permanent magnet type, connected in parallel with a shunt type AC regulator and a lighting system, and in some cases other electrical loads. Regulators of this type control the alternating current root mean square voltage of the alternator. A typical example of such a regulator would be found in Applicant's U.S. Pat. No. 3,755,709. It is common practice on these vehicles, and sometimes required by regulations, that the head lamps and tail lamps on the vehicles be illuminated whenever the engine is running. Therefore light switches, other than for selecting high and low beam, are not included. In the simplest of these vehicles there may be no other electrical components other than the alternator, voltage regulator and lights already mentioned. This existing configuration can be seen in the circuit diagram shown as FIG. 1. Even though the vehicle manufacturer supplies the vehicle without a light switch, some customers may object to the lights being on all the time, and ignoring safety implications, install their own switches. If a permanent magnet alternator of the type commonly used on these vehicles is operated open circuit or with little electrical load, the alternator voltages can be quite high at maximum engine speed. Typically an alternator that might be used in a nominal 12 volt electrical system could produce 100 volts if the voltage regulator and the other loads were removed. Thus in FIG. 1, if a vehicle owner installed a light switch at point C only the lights would be turned off when the switch was open and the voltage regulator would still control the voltage to the other electrical loads. However if the switch were installed at location B, any time the lights were turned off the regulator would be disconnected from the alternator, therefore full alternator open circuit voltage would be across small electrical loads that might be connected at point A. If no other electrical loads were present, the electrical system would appear to work normally even with a switch installed at point C.
It should be realized also that other failures in wiring or of other components in the electrical system might also produce high voltages on a portion of the electrical system where other electrical loads might be connected. FIG. 2 shows a circuit diagram and a portion of the mechanical parts of a fuel gauge system of a type well known and often used in direct current automotive applications. The system consists of an indicator needle attached to a pivot and also to a permanent magnet. Two electrical windings, shown as L1 and L2, supply a magnetic field around the permanent magnet. These windings are normally mechanically placed so that they are approximately 90 degrees apart. One of the 2 windings has a variable resistor, shown as R1, externally connected in series. This resistor is normally within the fuel tank and varied by a float. The float position changes with the level of fuel in the tank. With R1 at a value quite high compared to the internal resistance of L2, the magnet would tend to align itself with the magnetic field of L1 which would correspond to one pointer location. At the other extreme float location with the resistance of R1 near zero, the internal resistances of L1 and L2 can be so designed that the magnet will align itself with the magnetic field of L2 moving the pointer to the other end of the scale. Intermediate positions of the resistor R1 will give intermediate positions of the pointer.
It is understood by those involved in these instruments that the position of the pointer is dependant on the direction of the vector sum of the magnetic fields of L1 and L2 and thus independent of the magnitude of the power supply voltage supplied from point D to ground. Therefore, the gauge can be made to read accurately whether the voltage at point D is a steady DC voltage or varying DC voltage such as would result from a rectified AC signal. The high limit of voltage useable at point D is determined by the maximum heating allowable in L1, L2 or R1.
Since R1 is mounted, normally as an exposed resistor within the fuel tank, overheating of this component could be extremely dangerous causing possible fire or explosion. The low limit of the voltage useable at point D would be when the mechanical friction in the pivot became significant compared to the attraction of the magnetic field from coils L1 and L2 to the magnet attached to the pivot. It is known that the magnetic force in a circuit of this type increases with the square of the magnetic field. Since the magnetic field involved is essentially proportional to the instantaneous voltage at point D, it can be understood that a pulsed DC voltage at point D is very effective in overcoming the mechanical friction associated with the pivot.
A first attempt at connecting the known gauge system as shown in FIG. 2 to the known electrical system as is shown in FIG. 1 might be to connect a diode directly from point A in FIG. 1 to point D in FIG. 2 with the anode of the diode to point A. This would indeed supply appropriate electrical voltages to the gauge system for proper operation assuming the gauge system was designed for a nominal 12-volt system and that the regulator in FIG. 1 was a nominal 12-volt RMS regulator. This connection however could be extremely dangerous as has already been discussed because various modifications or failures in the electrical system as shown in FIG. 1 can produce high voltages at point A. If this voltage is connected through a diode to point D, the resistor R1 in the fuel tank could easily become incandescent, igniting the fuel in the tank with disastrous results.
It is an object of this invention to produce an electrical system which may be safely connected from point A of FIG. 1 to point D of FIG. 2. This connection between two known circuits must supply the required power for the gauge of FIG. 2 while protecting the gauge of FIG. 2 from destructive voltages even if the circuit of FIG. 1 is open at point B.
It is a further object to the present invention to do this without the use of components such as fuses which would be blown by a failure such as the inclusion of a switch at point B.
It is a further object of this invention to use components in this invention in such a way that failures of those components, in a method that is reasonable to expect, will not result in excessive or dangerous voltage being applied to the gauge system of FIG. 2.
It is a further object to this invention to accomplish the forgoing in the smallest package possible with a minimum number leads thus reducing the likelihood of incorrect connection of those leads.
It is a further object of this invention to produce an assembly that even if the leads are reversed or otherwise missconnected, dangerous voltages will not be produced across resistor R1.
It is a further object of this invention to minimize the power dissipation of this assembly even under fault conditions such as when a switch is open at location B.
The above and other objects of this invention are achieved in the rectification and regulation system as shown in FIG. 3. The components of this system allow for the rectification of power from point A primarily by diode D1, and the flow of that power to point D through resistor R2 under normal operating conditions. Under these conditions as will later be described, the transistor Q1 is turned on whenever point A is positive with respect to point D. When turned on, the resistance of Q1, shown as a N channel mosfet, is low in comparison to R1, R2, and the resistance of L1 and L2. Transistor Q1 and the other electrical components are however arranged so that if excessive voltage is present at point A, the current through this circuit to point D is limited to a predetermined maximum value during one quarter of the electrical cycle, and to zero during the remaining three quarters of the electrical cycle. Under these conditions resistor R2 serves as a current shunt to control transistor Q1. Thus the circuit prevents excessive or dangerous power from being applied to the gauge system of FIG. 2 and also protects it's own internal components.